Method of detecting pinhole defects in sheet material

ABSTRACT

Method of detecting pinhole defects in sheet materials for example tin plate, galvanized steel and thin metallic foils. A modulated ultra-violet light source is directed at right angles onto the advancing sheet material, the edges of the sheet material are shielded from both ambient and ultra-violet light, all non-ultra-violet light is shielded out from beneath the advancing strip, ultra-violet light which has been transmitted through the pinhole defects and said filtering is photomultiplied as a function of pinhole defects in the advancing material. The method is distinguished from prior art in collimating the ultraviolet light source so as to transmit the light at a right angle to the direction of motion of the sheet material and independently baffling the photomultiplier units with respect to each other, so as to define precise lineal zones of detection in said advancing material.

United States Patent [19] Murray et a1.

[451 Aug. 28, 1973 METHOD OF DETECTING PINHOLE DEFECTS IN SHEET MATERIAL[73] Assignee: Columbia Research Corporation,

Gaithersburg, Md.

[22] Filed: Mar. 9, 1972 [21] Appl. No.: 233,240

Primary Examiner-Archie R. Borchelt Attorney-David l-l. Semmes [5 7]ABSTRACT Method of detecting pinhole defects in sheet materials forexample tin plate, galvanized steel andthin metallic foils. A modulatedultra-violet light source is directed at right angles onto the advancingsheet material, the edges of the sheet material are shielded from bothambient and ultra-violet light, all non-ultra-violet light is shieldedout from beneath the advancing strip, ultraviolet light which has beentransmitted through the pinhole defects and said filtering isphotomultiplied as a function of pinhole defects in the advancingmaterial, The method is distinguished from prior art in collimating theultra-violet light source so as to transmit the light at a right angleto the direction of motion of the sheet material and independentlybaffling the photomultiplier units with respect to each other, so as todefine precise lineal zones of detection in said advancing material.

8 Claims, 1 Drawing Figure //LK3HT SOURCE r10 PARTITIONS PHOTOMULTIPUERTUBE Pmmcowszem 3.755674 LIGHT SOURCE OLLIMATOR UV FILTER g GLASS 34?36A 38* 40 42 44 e 20 72 74 e s W 2% vw PRE- PRE- PRE- PRE- PRE- PR'-ADJUSTABLE MP AMP AMP AMP AMP AMP fififgg- PHOTOMULTIPLIER TUBE CONTROLUNIT 74 AMP oNE FOR DET EACH CHANNEL OUNTER METHOD OF DETECTING PINHOLEDEFECTS IN SHEET MATERIAL CROSS REFERENCES TO RELATED APPLICATIONS Thisapplication claims priority of an earlier filed Great Britainapplication entitled Apparatus for Detecting Defects in Sheet Material(U.S. Pat. Ser. No. 05325/71), filed May 21, 1971.

BACKGROUND OF THE INVENTION 1. Field of the Invention Detection ofpinhole defects in advancing sheet material. Earlier inventors havedirected ultra-violet light against advancing sheet material, positioneda filter beneath the materials so as to filter out all non-ultra-violetlight and supported a plurality of photosensitive means, for instance,photomultiplier tubes beneath the filter to develop electrical signalsas a function of that ultraviolet light which has been transmittedthrough pinhole defects in the advancing sheet material.

2. Description of the Prior Art Linderman, US. Pat. No. 2,758,712.

This patent is typical of the prior art wherein a plurality ofphotomultiplier tubes are positioned beneath an ultra-violet lightsource and advancing sheet material. Since the ultra-violet light sourceis elongated in nature, and the photomultiplier tubes are not baffled orshielded with respect to each other, it is not possible to laterallypinpoint the area of these defects which have been detected in theadvancing sheet material. For example, the ultra-violet light might bestriking the sheet material at an extreme acute angle and be picked upby a photomultiplier tube laterally displaced from the pinhole defect.Also, there is not means for protecting the ultra-violet filter from thedeleterious effects of dust and grime which adhere in most milloperations.

SUMMARY OF THE INVENTION According to the present method, the sheetmaterial is advanced through a detection plane, a modulated ultra-violetsource is directed onto the plane. The ultraviolet light is collimatedin a series of vertical channels, such that the light strikes the sheetmaterial perpendicularly to the width of the material, the edges of theadvancing sheet material are shielded from both ultraviolet and ambientlight, non-ultra-violet light is filtered out from beneath the materialand ultra-violet light transmitted through defects in the sheet materialis photomultiplied and transformed into electrical pulses, the pulses,being measured and accumulated as a function of pinhole defects in theadvancing sheet material. According to a modification of the invention,the filtering area beneath the advancing material may be protected fromdust and oil deposits by transversely advancing a plastic film over thearea of filtering.

BRIEF DESCRIPTION OF THE DRAWINGS The FIGURE is a schematic view of theproposed method showing suggested light source, collimator, advancingstrip, filter, photomultiplying, amplifying and detecting components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention relates to amethod for detecting defects in advancing sheet material, and isparticularly concerned with the detection of small voids or apertures,hereafter referred to as pinholes," in sheet metals such as tin plate,galvanized steel, tin-free steel, and especially thin metallic foilsmade, for example, of copper, aluminum, or any opaque thin stripmaterial such as plastics, the apparatus being particularly applicablewhere the sheet material is slit longitudinally into strips or bands.

According to the method, apparatus for detecting defects in sheetmaterial comprises an ultra-violet light source arranged on one side ofsaid material and means to direct beams of light on the surface of saidmaterial, said apparatus also being provided with light responsive meansarranged on the opposite side of said material, said light receivingmeans including photosensitive means activated by beams of light passingthrough said material to generate signals thereby, signal processingmeans also being provided in said apparatus to detect the presence ofholes and differentiate the signals from electrical and ambient noise.Several light responsive means may be used and the signal from eachmeans processed independently so as to give an indication of theirposition across the width of the strip.

In an embodiment of the invention, one example of which is shown in theaccompanying sketch, a suggested apparatus might consist of a controlunit 4 which contains the necessary power supply means and regulatorsfor the ultraviolet light source 10 and the light responsive means andan electronic detector. Connected to the control unit, and placed on theproduction line of the material to be inspected, in this example amoving strip of brass foil 22, is the sensor unit, which comprises alamp housing containing a light source 10 and located above the stripand a scanner housing 6 which includes a plurality of photomultipliers70, 72, 74, 76, 78 and 80 located beneath the strip 22 being monitored.

Light source 10 is preferably an ultra-violet fluorescent lamp and theapparatus operates by sensing the passage of ultra-violet beams of lightpassing through flaws in the strip and falling on the photomultipliers.Each beam generates a signal which is processed and used, for example,to operate counting means, recording means, or other quality controldevices such as alarms.

False signals due to ambient illumination are eliminated by providing asystem of light shields 24 and 26 to prevent ambient, as well asdirected light, from reaching the photomultipliers through a path aroundthe edge of the strip. To assist further indiscrimination againstambient light, light source 10 is modulated at a frequency considerablyhigher than those encountered in the plants. This could be, forinstance, 8,000 cycles. Each beam of ultra-violet light passing througha hole in the strip enters the scanner housing 6 and thence the detectorcircuitry through an ultra-violet bandpass filter 28. The detectorcircuit is sensitive only to the modulation frequency and, in addition,conventional electrical filtering means may be provided which, with theoptical filters, ensures good signal-to-noise ratios.

Preferably, a collimator 2, including baffles 12, 14, 16, 18 and 20, isinterposed between light source 10 and the facing surface of thestrip 22to ensure that the beams of modulated ultra-violet light strike thematerial perpendicularly to the width of the strip material, and toensure that each beam of light coming through a pinhole can only reach asingle photomultiplier tube.

The scanner housing is preferably provided with a filtering window 28adjacent to the surface of the strip, the window being a filter whichallows only the chosen part of the light spectrum to pass into thedetector circult. Thus, in the case of modulated ultra-violet light,only this light will be able to reach the detector circuit.

Below the window is a plurality of photomultiplier tubes 70, 72, 74 76,78 and 80 positioned between externally adjustable baffles 32, 34, 36,38, 40, 42 and 44 arranged across the width of the strip. The entryaperture for the sensed light comprises a narrow and externallyadjustable slot extending in the same direction as the collimator. Theindividual photomultiplier tubes are connected to preamplifiers, andsignals generated by the photomultiplier tubes are fed back to thecontrol unit 4, embodying conventional amplifying, pulse detecting andcounting elements. Each detected-signal corresponds to a hole in thestrip and provides both a count of the holes and indication of theposition of holes with respect to the width of the strip.

The light which enters the scanner box 6 in which the photosensitivemeans are located is divided into discrete portions by the partitionswhich are placed between the photosensitive elements and by the placingof baffles, traps, etc., which prevent the light from getting from onephotosensitive device to the others. Both the strip and the partitionsare kept close to the ultraviolet window 28 to maintain minimum opticalscattering between the photosensitive device or devices being used todefine different areas across the width of the strip.

The signals are processed in the electronic detector unit and may becounted by suitable means, provided that consecutive holes are not tooclose together.

In addition to the operation of simple counting means or alarms, thesignal processing detector unit can be arranged to classify and/or countpinholes of different sizes, for example by discriminating as to pulseheight. In this way, a large number of pinholes or similar defects canbe detected and counted, either in total and- /or in groups according tosize. A permanent record of the defects is also possible, and theapparatus may be provided with means to indicate the location ofpinholes on the strip itself in order that the faulty length may belocated.

As has been stated, the light is preferably ultra-violet which ismodulated at a suitably high frequency. The apparatus may, however, bemodified to operate with other parts of the spectrum, and the frequencymodulation may be omitted. Other collimating means which may be used maybe a series of lenses, each associated with a light source of smalldimension located at its focus so as to obtain parallel rays of lightperpendicular to the width of the sheet material. In a modifiedembodiment of the invention, edge shielding means 24 and 26 areprovided. Preferably, the edge shielding means do not contact thematerial and are arranged to The apparatus is primarily designed for usein slitting lines, the apparatus then being used to detect the position,size and/or number of pinholes in each band 82, 84, 86, 88, 90 and 92into which the strip 22 is slit. The vertical partitions or bafflesmentioned above being disposed at the same position across the width ofthe strip as are the slitter knives 60, 62, 64, 66 and 68. The apparatusmay also be provided with means for discriminating between pinholes andedge cracks or tears, the length of the crack or tear being used toenable discrimination between these flaws and pinholes which wouldotherwise lead to false indications. Test lamps 46, 48, 50, 52, 54 and56 would also be provided if desired to check the operation of eachphotomultiplier and its associated circuit.

We claim:

1. Method of detecting pinhole defects in sheet material comprising:

A. advancing said sheet material through a detection plane;

B. directing ultra-violet light onto said plane and onto said advancingsheet material and through any pinholes existing therein; saidultra-violet light being modulated at a frequency substantiallyexceeding that of commercial lighting frequencies;

C. collimating said ultra-violet light in a series of vertical channelsby means of partitions perpendicular to the width of the sheet materialso as to cause to reach the strip only that portion of the light whichis perpendicular to the width of the sheet material;

D. shielding both edges of said advancing sheet material from bothultra-violet and ambient light;

E. filtering out non-ultra-violet light from beneath said advancingmaterial;

F. impinging the modulated light which has been transmitted through saidpinhole defects onto a plurality of separate photomultiplier tubespositioned beneath said detecting plane as a laterally disposed seriesextending across the width of the sheet and shielded from and withrespect to one another by vertical partitions so as to receivetransmitted light within a confined area;

G. transforming the modulated light which has been transmitted throughsaid pinhole defects and said filtering and onto said photomultipliertubes into a series of electrical pulses; and accumulating and measuringsaid pulses as a function of pinhole defects in said advancing sheetmaterial.

2. Method of detecting pinhole defects in sheet material as in claim 1,wherein said ultra-violet light source is modulated at a frequencyexceeding 500 cycles per second.

control light shields or traps placed at either edge of the strip toprevent light from being able to pass round the strip edge and into thelight sensitive device circuit.

The apparatus may also have means associated with the window throughwhich light enters the detector stage by which the window is kept cleanand dust-free. This means may, for example, consist of a strip oftransparent film 30 passing over the detector window, the rate at whichthe strip passes over the window being variable to match operatingconditions.

' lineal detection zones of varying widths within said detection plane.

5. Method of detecting pinhole defects in sheet material as in claim 4,including longitudinally cutting said advancing sheet material intostrips adjacent said pholomultiplying and conforming the width of saidshielding to the width of said strips.

6. Method of detecting pinhole defects in sheet material as in claim 5,including cleaning while filtering by moving a transparent filmintermediate the bottom of said advancing sheet and the area of saidfiltering.

6 plier signals as a series of electrical pulses.

8. Method of detecting pinhole defects in sheet material as in claim 7,including measuring pulse height as 7. Method of detecting pinholedefects in sheet mate- 5 a function of Pinhole defect sizerial as inclaim 6, including amplifying said photomulti-

1. Method of detecting pinhole defects in sheet material comprising: A.advancing said sheet material through a detection plane; B. directingultra-violet light onto said plane and onto said advancing sheetmaterial and through any pinholes existing therein; said ultra-violetlight being modulated at a frequency substantially exceeding that ofcommercial lighting frequencies; C. collimating said ultra-violet lightin a series of vertical channels by means of partitions perpendicular tothe width of the sheet material so as to cause to reach the strip onlythat portion of the light which is perpendicular to the width of thesheet material; D. shielding both edges of said advancing sheet materialfrom both ultra-violet and ambient light; E. filtering outnon-ultra-violet light from beneath said advancing material; F.impinging the modulated light which has been transmitted through saidpinhole defects onto a plurality of separate photomultiplier tubespositioned beneath said detecting plane as a laterally disposed seriesextending across the width of the sheet and shielded from and withrespect to one another by vertical partitions so as to receivetransmitted light within a confined area; G. transforming the modulatedlight which has been transmitted through said pinhole defects and saidfiltering and onto said photomultiplier tubes into a series ofelectrical pulses; and accumulating and measuring said pulses as afunction of pinhole defects in said advancing sheet material.
 2. Methodof detecting pinhole defects in sheet material as in claim 1, whereinsaid ultra-violet light source is modulated at a frequency exceeding 500cycles per second.
 3. Method of detecting pinhole defects in sheetmaterial as in claim 1, including testing said photomultiplier tubes byoriginating light adjacent each said tube beneath said plane and withinsaid confined area.
 4. Method of detecting pinhole defects in sheetmaterial as in claim 3, including laterally adjusting said shielding ofsaid photomultiplier tubes so as to define lineal detection zones ofvarying widths within said detection plane.
 5. Method of detectingpinhole defects in sheet material as in claim 4, includinglongitudinally cutting said advancing sheet material into stripsadjacent said pholomultiplying and conforming the width of saidshielding to the width of said strips.
 6. Method of detecting pinholedefects in sheet material as in claim 5, including cleaning whilefiltering by moving a transparent film intermediate the bottom of saidadvancing sheet and the area of said filtering.
 7. Method of detectingpinhole defects in sheet material as in claim 6, including amplifyingsaid photomultiplier signals as a series of electrical pulses.
 8. Methodof detecting pinhole defects in sheet material as in claim 7, includingmeasuring pulse height as a function of pinhole defect size.